The present invention relates to methods for closing holes in components that operate at high temperatures, such as holes located at the tips of gas turbine buckets. More particularly, this invention relates to a process of filling holes in castings formed of alloys that are prone to strain age cracking when attempting to fill such holes by conventional welding techniques.
Components of gas turbines, such as buckets (blades), nozzles (vanes), and other hot gas path components, are typically formed of nickel, cobalt or iron-base superalloys with desirable mechanical and environmental properties for turbine operating temperatures and conditions. Because the efficiency of a gas turbine is dependent on its operating temperatures, there is a demand for components, and particularly turbine buckets and nozzles, that are capable of withstanding increasingly higher temperatures. As the maximum local temperature of a superalloy component approaches the melting temperature of the superalloy, forced air cooling becomes necessary. For this reason, airfoils of gas turbine buckets and nozzles often require complex cooling schemes in which air is forced through internal cooling passages within the airfoil and then discharged through cooling holes at the airfoil surface.
Buckets and nozzles formed by casting processes require cores to define the internal cooling passages. During the casting process, shifting of the cores is prevented by supporting the cores within the mold using quartz rods or similar means, which often result in openings (through-holes) in the casting in the region of the bucket tip. These openings must be securely closed or plugged to prevent the loss of cooling air through these openings and ensure proper air flow levels through the intended cooling holes of the casting. Various methods have been used to fill these openings, including brazing and welding techniques, the latter of which includes tungsten inert gas (TIG) welding, electron beam welding, and laser beam welding. As an example, openings have been sealed with a cover plate through welding or brazing processes during post cast-operations. In some cases, welding is not practical for closing or filling holes due to costs, poor fusion weldability of the material, or restrictions arising from the configuration of the component. Furthermore, welding techniques involve application of localized heat energy that produces a fusion zone and a base metal heat-affected zone (HAZ) that are prone to liquation and strain age cracking.
Particularly notable alloys that have found wide use for gas turbine buckets include the gamma prime-strengthened (principally Ni3(Al,Ti)) nickel-base alloys GTD-111® and René N5, which are high strength and oxidation-resistant superalloys often produced as directionally-solidified (DS) and single-crystal (SX) castings for gas turbine applications. GTD-111® has a nominal composition, by weight, of about 14.0% Cr, about 9.5% Co, about 3.0% Al, about 4.9% Ti, about 1.5% Mo, about 3.8% W, about 2.8% Ta, about 0.010% C, the balance nickel and incidental impurities, and N5 has a nominal composition of, by weight, about 7.5% Co, about 7.0% Cr, about 6.5% Ta, about 6.2% Al, about 5.0% W, about 3.0% Re, about 1.5% Mo, about 0.15% Hf, about 0.05% C, about 0.004% B, about 0.01% Y, the balance nickel and incidental impurities. Buckets produced from these alloys have been found to be particularly prone to cracking due to their chemical compositions, and particularly their high volume fraction of gamma prime attributable to the combined amounts of titanium and aluminum in these alloys (greater than five weight percent Ti+Al). As known in the art, when components made from precipitation-hardened alloys are welded, gamma prime (γ′) and gamma double prime (γ″) phases are dissolved in and near the weld. When the component later experiences sufficiently high temperatures, these strengthening phases can reprecipitate more rapidly than the relaxation of residual stresses remaining from the welding process. The weld and surrounding area are thus incapable of accommodating the strains required to relieve the residual stresses, with the result that the weld and/or heat-affected zone may crack.
One approach to mitigate or eliminate the tendency for welding cracking is to select a cover material that exhibits better weldability, typically as a result of containing a combined amount of titanium and aluminum of less than the bucket alloy being welded, for example, less than five weight percent Ti+Al. A notable example of such a superalloy is the gamma prime-strengthened nickel-base superalloy GTD-222® having a nominal composition, in weight percent, of about 22.5% Cr, about 19.0% Co, about 2.3% Ti, about 1.2% Al (about 3.5% Ti+Al), about 2.0% W, about 0.8% Nb, about 1.0% Ta, about 0.01% Zr, about 0.01% B, about 0.1% C, with the balance being nickel and incidental impurities. While meeting the weldability requirements for closing bucket tip openings, GTD-222® has lower mechanical properties as compared to GTD-111®. Furthermore, prior welding techniques using weldable superalloys such as GTD-222® have not entirely avoided the occurrence of strain age cracking.